Aperture arrangements are used in light and charged particle optical systems to reduce beam divergence angles and beam intensity through the system. Apertures can also be used to intercept unused portions of the beam to prevent undesirable beam interactions with the optical system.
The ability to control beam divergence angle is important because it affects the magnitude of aberrations in the optical system. Such aberrations can cause an undesirable increase in beam spot size. The ability to change aperture size is also useful for controlling beam intensity. This may be required where a sample is sensitive to irradiation and reduced intensity is required. Alternatively, signal acquisition times may be critical in which case a higher intensity may be required. In a charged particle system, there is a trade-off between beam intensity and spot size, greater intensity necessarily resulting in a greater spot size for non-diffraction limited systems (i.e. most systems for e-beam testing and lithography). The ability to adjust the aperture is one method by which a user can tune the optical performance of a system (intensity vs. spot size) according to their particular requirements.
Two approaches have been previously proposed to allow variation of the aperture of a system, iris diaphragms and multiple fixed apertures of varying sizes.
Iris diaphragms have been used traditionally in light systems and include a single aperture with an iris mechanism which can be used to reduce the opening of the aperture. Iris diaphragms are usually limited to minimum aperture sizes of about 500 .mu.m diameter and aperture member thickness of about 100 .mu.m. For charged particle systems, apertures as small as 10 .mu.m diameter and 1 .mu.m thickness may be required to restrict the acceptance angle sufficiently. This effectively rules out the ability to use iris diaphragms.
An alternative mechanism used in such circumstances comprises a strip with a discrete number of fixed size apertures which may be positioned in the beam. Because no moving parts are involved in the apertures themselves, small diameters and thicknesses are possible. To change apertures, two approaches are possible. Either the strip is translated across the beam until the desired aperture is aligned with the beam, or the beam itself is deflected so as to pass through the desired aperture.
With either mechanism only discrete changes in aperture sizes, and, hence acceptance angles, are possible unless they are used in combination with other changes to the charged particle (electron) optical system such as changing lens excitations. However, changing lens excitation (focal length) changes the magnification of that lens. This leads to unwanted coupling between changes in (effective) aperture size and changes in electron optics.
It is an object of the present invention to provide an optical system in which the aperture is continuously variable and which does not suffer from the problems associated with previous approaches.